Biaxially oriented polyester films are excellent in thermal properties, dimensional stability, mechanical properties, electric properties, heat resistance and surface properties, and therefore are used for various industrial materials such as magnetic recording media, electric insulation, capacitors and packaging. In particular, it is well known that biaxially oriented polyester films are useful as substrate of magnetic recording media, etc.
In recent years, magnetic recording media such as magnetic tapes are required to have thinner base films and allow higher density recording since machines and materials are required to be lighter in weight, smaller in size and larger in capacity. In order to allow higher density recording, it is effective to employ shorter recording waves and smaller recording tracks.
However, a smaller recording track has a problem that the recording track is likely to shift owing to the deformation of the tape caused by the heat during tape running and by the changes of temperature and humidity during tape storage. Consequently there is a growing demand for a base film with higher dimensional stability even in the tape use environment and the tape storage environment. Further, there is also a growing demand for a magnetic tape with higher running durability.
Furthermore, a thinner film is insufficient in mechanical strength, hence less stiff and likely to elongate in the machine direction and likely to contract in the transverse direction. Consequently it has such problems that the track shifting occurs, that the head touch is adversely affected to lower the electromagnetic conversion properties, and that the head and the tape are abraded.
From these points of view, aromatic polyamides more excellent in strength and dimensional stability and higher in stiffness than biaxially oriented polyester films are used sometimes. However, since the stiffness of the aromatic polyamides is too high, the head may be abraded. Further, since the aromatic polyamides are expensive, it is not realistic to use the aromatic polyamides as substrate of general-purpose recording media. Polyester films formed from polyethylene terephthalate, polyethylene naphthalate and the like are also used to develop substrate with higher strength for magnetic recording media using stretching techniques. However, it is still difficult to satisfy the severe demand for dimensional stability to temperatures and humidities.
In recent years, in order to enhance the heat resistance of a polyester film, methods of blending a polyester and another thermoplastic resin are studied.
A biaxially oriented polyester film obtained by mixing a polyester and a thermoplastic resin other than a polyester is proposed as a film excellent in running properties and scratch resistance (for example, patent document 1). However, this technique is intended to improve the scratch resistance of the surface of a film, and is different from the present application in technical idea. Actually the technique described in the document cannot enhance mechanical properties or dimensional stability. Further, the document does not disclose the mixing method important for enhancing the dimensional stability of a film as disclosed in the present application, when mixing a thermoplastic resin other than a polyester into a polyester, or the particular film forming method for preparing a film using three resin components such as a polyimide described in the examples of the present application.
Further, proposed is a film consisting of a polyester, a polyimide and a polymer nano-compatible with the polyimide, in which an aromatic polyether ketone or the like is used as the polymer nano-compatible with the polyimide, to enhance heat resistance and thermal dimensional stability (for example, patent document 2). However, in this technique, the amounts of the polyimide and the polymer nano-compatible with the polyimide, mixed with the polyester are so large that the molecular chains may not be able to be oriented effectively by stretching or the like. For example, in the case where the film is used for a higher density magnetic recording medium or the like, the film may not be able to satisfy the severe demand for the dimensional stability to temperatures and humidities, etc. Further, foreign object owing to the non-molten material is likely to be generated in the film, to roughen the surface, and in the case where the film is used, for example, for a magnetic recording medium, electromagnetic conversion properties may become poor.
Moreover, a resin composition composed of a polyimide and a thermoplastic resin other than a polyimide is proposed (for example, patent document 3). However, for example, any specific method for applying the resin composition to a polyester film is not disclosed at all.